1. Field of the Invention
The invention relates in general to a method and associated apparatus of communication parameter detection, and more particularly to a method and associated apparatus of communication parameter detection for detecting a temporal length of a guard interval based on a histogram and capable of detecting whether a communication signal carries valid information.
2. Description of the Related Art
Various communication systems, particularly wireless communication systems, are prevalent in the modern information society. For example, a wireless communication system based on an orthogonal frequency division multiplexing (OFDM) technique is a common wireless communication system. Specifications from IEEE 802.11a wireless local area network standards, IEEE 802.16 wireless metropolitan network standards to Digital Video Broadcasting (DVB) standards developed in the Euro zone all implement OFDM techniques.
In an OFDM wireless communication system, to transmit a radio frequency (RF) communication signal for transmission or broadcasting, information to be transmitted (or broadcasted) is coded and constellation mapped to carry the information in a series of complex frequency domain coefficients of orthogonal carriers. In equivalence, a complex inverse frequency domain transform (e.g., inverse Fast Fourier Transform (FFT)) is performed on the coded and constellation mapped information to be transmitted to form a symbol in a baseband communication signal. For protection against inter-symbol interference (ISI) resulted by multipath transmissions in a wireless transmission environment, between every two symbols of a baseband communication signal is provided with a guard interval. Each guard interval is filled with a cyclic message, e.g., a cyclic prefix (CP). A cyclic prefix of a symbol is a repetition of a last part of the symbol. That is, a length at the last part of the symbol that equals a part as the guard interval is filled to the guard interval to serve as a cyclic prefix of the symbol. The baseband communication signal added with the cyclic message may be up-converted to an RF communication signal and then to an analog wireless signal, which is transmitted in form of a wireless wave.
In an OFDM wireless communication system, corresponding processes are performed when a receiver receives the analog wireless signal transmitted from the transmitter. More specifically, the analog wireless signal is down-converted to/sampled as a digital discrete baseband communication signal, from which the cyclic message is removed. Complex frequency domain transform (e.g., FFT) is performed followed by constellation inverse-mapping and decoding, so as to retrieve the information carried in the communication signal.
To adapt to different wireless communication environments, an OFDM wireless communication system may adopt communication signals of different modes. In the communication signals of different modes, a temporal length between two consecutive cyclic messages may vary. Further, even for communication signals of a same mode, a temporal length of the guard interval may also be selected from multiple possible candidate guard intervals.
For example, communication signals exist in 2K, 4K and 8K modes. Under the 2K mode, the time (to be referred to as a first period L1) between two consecutive cyclic messages covers 2048 sampling points in the communication signal. Similarly, under the 4K and 8K modes, the first period respectively covers 4096 and 8192 sampling points. Under a same mode, the period (to be referred to as a second period L2) of a guard interval may be 1/32, 1/16, ⅛ or ¼ of the first period L1. In other words, the transmitter may select one from the candidate guard intervals L1/32, L1/16, L1/8 and L1/4 to accordingly set the temporal length of the guard interval.
The transmitter does not clearly inform the receiver of the adopted mode and the temporal length of the guard interval. Thus, to detect the mode and the temporal length of the guard interval of the received communication signal, it is necessary that the receiver perform a blind test for communication parameters according to the received communication signal. Only based on known mode and temporal length of the guard interval of the communication signal, the receiver can correctly remove the cyclic messages and perform frequency domain transform to correctly retrieve the information in the communication signal.
In a conventional technique of communication parameter detection, the receiver exhaustively tests the communication signal with respect to all candidate guard intervals under all modes one after another to determine whether the communication signal matches any of the candidates. More specifically, assuming the communication system has N_of_mode number of modes each having N_of_GI guard intervals, the convention technique performs N_of_mode*N_of_GI tests on the communication signal to confirm the mode and the guard interval of the communication signal. It is apparent that such exhaustive test process consumes a great amount of time for the communication parameter detection and hence disfavors the communication efficiency.
Further, in the event that the transmitter does not designate any meaningful, valid information to the communication signal, the communication signal received at the receiver contains only noise. At this point, the receiver is expected to recognize that the communication signal does not contain valid information. However, in the conventional technique, valid information detection cannot be concurrently carried out with the communication parameter detection. Only after the communication parameters are detected, the convention technique is then able to determine whether the information is valid or meaningless according the information obtained from frequency domain transform, inverse mapping and decoding. Hence, the conventional technique fails in promptly determining the validity of the communication signal.